The present invention is directed to auxiliary power units having fire enclosures. More particularly, the invention relates to seals for joining drains to fire enclosures in the auxiliary power unit.
Auxiliary power units (APUs) comprise gas turbine engines that operate to provide various power inputs to aircraft, such as helicopters, when the main propulsion engines are not operating, such as during ground operations or during the event of an outage during flight. APUs can additionally provide supplemental power to that generated during main engine operations. APUs typically comprise gas turbine engines having a compressor and a turbine, between which a combustor burns fuel. Through a gearbox, the turbine provides mechanical input to an electrical generator, while compressed air bled from the compressor is used to supply various environmental control systems.
APUs are typically located within the outer skin of the fuselage of the aircraft. Thus, it is desirable to encapsulate hot sections of the APU to provide a fire break where fuel is present. Conventional practice, such as is described in U.S. Pat. No. 7,526,921 to Williams et al., involves rigidly bolting a fire enclosure to various fixed positions on the APU. Temperature variations that arise during different operating cycles of the APU produce thermal expansions of various APU components that alter the distances between the fixed positions. Thermal growth of the APU thus induces strain into the fire enclosure. The ability of the fire enclosure to tolerate deflection or bending is limited because the fire enclosure is not a structural component designed to absorb loading.
Furthermore, it is desirable to be able to drain fuel from the combustor out of the fire enclosure in the event of unburned fuel being present in the combustor, such as from a failed start or some other such occurrence. Previous attempts at providing drains on APU fire enclosures involved using check valves that needed to be actively closed to prevent combustor air from escaping during operation of the APU and then opened to drain fuel. These valves, however, typically failed to a closed position, which could lead to fuel pooling within the combustor causing a fire hazard. Other fire enclosures, such as described in the aforementioned patent to Williams et al., involve bleed air ports having bulb seals that require precise alignment of parts. There is, therefore, a need for a fire enclosure drain that can accommodate thermal growths and misalignments within APU fire enclosures.